usbnet: Omit needless tests in usbnet_tick.

It's harmless for us to schedule the tick task even if unp_dying or
unp_stopping is set by now, because usbnet_stop will just wait for it
to finish anyway, and the callout can't be scheduled again until the
interface is done stopping and is brought back up again.

No need for unp == NULL test -- un->un_pri is initialized well before
this callout can be scheduled, and is nulled out only at the end of
usbnet_detach, at which point we have already halted this callout.

(riastradh)